U.S. patent application Ser. No. 07/727,806, filed Jul. 8, 1991, discloses a process for producing in-situ heat treating atmospheres from non-cryogenically generated nitrogen. According to this patent application, suitable atmospheres are produced by 1) mixing non-cryogenically generated nitrogen containing up to 5 vol. % residual oxygen with a reducing gas such as hydrogen, 2) feeding the gaseous mixture into a furnace in a specified manner to effect conversion of the residual oxygen to an acceptable form such as moisture. The flow rate of hydrogen according to the application is controlled in such way so that it is always greater than the stoichiometric amount of hydrogen required for the complete conversion of residual oxygen to moisture. Specifically, the flow rate of hydrogen for oxide annealing is controlled between 1.1 times to 1.5 times the stoichiometric amount. Likewise, the flow rate of hydrogen for decarburize, bright annealing is controlled to be at least 3.0 times the stoichiometric amount.
The residual oxygen present in non-cryogenically generated nitrogen is reacted with hydrogen and converted to moisture following the equation: EQU 2H.sub.2 +O.sub.2 .fwdarw.2H.sub.2 O
According to this equation, two moles (or parts) of hydrogen react with one mole (or part) of oxygen to yield two moles (or parts) of water or moisture. For example, 0.5 vol. % residual oxygen present in non-cryogenically generated nitrogen requires a minimum of 1.0 vol. % hydrogen to produce 1.0 vol. % moisture or nitrogen gas with approximately 45.degree. F. dew point. One can therefore easily calculate the stoichiometric amount of hydrogen and that required for oxide and decarburize, bright annealing carbon steels knowing the level of residual oxygen in the feed gas. These values were calculated and are summarized below.
______________________________________ Stoichio. Oxide Decarburize, Bright Residual Amount Annealing Annealing Oxygen, % of H.sub.2, % H.sub.2, % D.P., .degree.F. H.sub.2, % D.P., .degree.F. ______________________________________ 0.2 0.4 0.44 22 1.2 22 0.5 1.0 1.10 45 3.0 45 1.0 2.0 2.20 62 6.0 62 1.5 3.0 3.30 76 9.0 76 ______________________________________
One can see that the stoichiometric amount of hydrogen and that required for oxide and decarburize, bright annealing carbon steels increase with the level of residual oxygen in non-cryogenically generated nitrogen.
It is well known in the literature that the thickness of an adherent, tightly packed oxide layer and the extent of decarburization of carbon steels depend on the temperature and the level of moisture present in the atmosphere. The thickness of oxide layer and the extent of decarburization increase with temperature and an increase in the moisture level in the atmosphere. Therefore, it is desirable to increase moisture level in the furnace atmosphere to produce parts with the required 1) thickness of the oxide layer and 2) level of decarburization.
According to the above patent application, if an atmosphere containing 1.0 vol. % moisture (or D.P. of 45.degree. F.) i s required for oxide annealing carbon steels, it is produced in-situ from non-cryogenically generated nitrogen containing 0.5% residual oxygen mixed with a slightly more than a stoichiometric amount (&gt;1.0%) of hydrogen. An atmosphere for decarburize, bright annealing carbon steels containing 1.0 vol. % moisture is produced from non-cryogenically generated nitrogen containing 0.5% residual oxygen mixed with at least 3.0% hydrogen. Likewise, if an atmosphere containing 3.0 vol. % moisture (or D.P. of 76.degree. F.) is required for oxide annealing carbon steels, it is produced in-situ from non-cryogenically generated nitrogen containing 1.5% residual oxygen mixed with a slightly more than stoichiometric amount (&gt;3.0%) of hydrogen. An atmosphere for decarburize, bright annealing carbon steels containing 3.0 vol. % moisture is produced from non-cryogenically generated nitrogen containing 1.5% residual oxygen mixed with at least 9.0% hydrogen. Therefore, it is clearly evident that the amount of hydrogen required for producing nitrogen-based atmospheres for oxide and decarburize, bright annealing carbon steels from non-cryogenically generated nitrogen increases with the level of residual oxygen in the feed stream. Therefore, it may not be economically feasible to produce high-moisture containing atmospheres from nitrogen feed stream with high-residual oxygen because of the excessive use of expensive hydrogen.
Based upon the above discussion, it is clear that there is a need to develop a process for producing high-moisture containing atmospheres suitable for oxide and decarburize annealing carbon steels economically from non-cryogenically generated nitrogen.